Method for regulating the evaporation rate of oxidizable substances in reactive vacuum deposition

ABSTRACT

A process for regulating the evaporation rate of oxidizable substances in reactive vacuum deposition by the metered addition of oxygen to a vacuum deposition chamber during deposition, wherein the addition of oxygen to a predetermined value, the pressure &#34;p&#34; in the vacuum chamber is measured, and the evaporation rate &#34;r&#34; for any pressure changes is regulated such that the ratio of pressure to evaporation rate p:r is maintained substantially constant.

BACKGROUND OF THE INVENTION

The invention relates to a method for regulating the evaporation rate ofoxidizable substances in reactive vacuum deposition by the meteredaddition of oxygen to a vacuum deposition chamber during deposition.

The quality, in particular the optical properties, of deposited oxidelayers depends to a very great extent in reactive evaporation on thehomogeneity of the layer material within the deposited layer. Thisapplies particularly to deposited multiple layer systems, to which theso-called interference layer systems belong. If it is not possible todeposit layer by layer under the same conditions, i.e. to apply eachlayer with the same constant composition and at approximately the samedeposition rate, then non-homogeneities arise, which adversely affectthe dependency of the absorption and transmission behaviour on the lightwavelength and in particular largely make it impossible to reproduce themethod. The absorption and transmission behaviour over specificwavelength regions of light can be shown diagrammatically, and one talksof absorption and transmission curves.

A basic requirement for uniformity within the individual layers is theuniform oxidation of the layer material by the oxygen introduced intothe vacuum chamber. Factors that interfere in the density or pressure ofthe oxygen in the vacuum deposition chamber are the suction capacitiesof the vacuum pumps connected to the vacuum chamber, the desorption ofgases, changes in the amount of oxygen added per unit time, as well asthe constant consumption of oxygen by the oxidation of the evaporant. Onaccount of the continuous addition of oxygen and the continuousevacuation by the vacuum pumps, an equilibrium state is established,which of course may be subject to variations over the course of time. Afluctuating oxygen partial pressure leads, in the case of a constantevaporation rate, i.e. in the liberation of uniform amounts of evaporantper unit time, to a varying oxidation behaviour of the evaporant, withthe result that the afore-described variations in the composition of thelayer material occur.

Those skilled in the art have therefore always attempted to maintain thefeed rate of oxygen to the vacuum chamber as constant as possible. Thisobjective can however be only very unsatisfactorily achieved and atconsiderable expense and effort, on account of the regulation behaviourof the metering valves required for this purpose. On the other hand,those skilled in the art also attempted to maintain the evaporation rateof the evaporation source as constant as possible by regulating theelectrical power. To this end, a whole number of so-called ratemeasuring devices were developed. However, the oxidation behaviournecessarily varied as a function of the amount of oxygen added per unittime.

SUMMARY OF THE INVENTION

The object of the invention is to provide a regulation method of thetype described at the beginning, by means of which single layers andmultiple layers of as homogeneous a degree of oxidation as possible canbe obtained.

This object is achieved with respect to the method of the presentinvention by adjusting the addition of oxygen to a predetermined value,measuring the pressure in the vacuum chamber, and regulating theevaporation rate for any pressure changes such that the ratio ofpressure to evaporation rate is maintained substantially constant.

The specified feature ensures that the ratio of oxygen per unit volumepresent in the vacuum chamber to the evaporation rate is maintainedconstant, i.e. the amount of evaporant released per unit time for theoxidation process is adapted to the amount of oxygen present. By virtueof the above-described equilibrium state, this also means that the ratioof the amount of oxygen added per unit time to the evaporation rate islikewise maintained constant. This constancy of the oxidation reactionleads to the desired homogeneous layers. As soon as the existingpressure measuring device establishes that the pressure in the vacuumchamber, preferably the oxygen partial pressure, falls, the evaporationrate is lowered by reducing the electrical power supplied, with theresult that the afore-mentioned ratio is maintained constant, with theexception of a slight control deviation. If on the other hand thepressure measuring device records a pressure rise in the vacuum chamber,in particular in the oxygen partial pressure, the evaporation rate isappropriately regulated upwardly, the desired constancy of theafore-mentioned ratio thereby likewise being achieved. Evaporationmaterials include not only metals, but also complete or partial oxidesof metals which would change as regards their chemical composition in aneutral atmosphere. The subject of the invention can be used inparticular for producing dielectric layers of TiO₂, SiO₂.

The desorption rate of the container or vessel should be as low aspossible. This is achieved by establishing a final vacuum that issmaller than 5×10⁻⁶ mbars, before introducing the oxygen.

The subject matter of the invention can be developed still further bytaking measures to ensure that the amount of oxygen is regulated to aconstant value per unit time. For this purpose, there already existmetering valves that can automatically regulate a constant throughput(weight amount) per unit time. At a constant suction capacity of thevacuum pumps the introduction of such a measure means that the pressurein the vacuum deposition chamber is now affected only by a varyingdeposition rate, with the result that the pressure-dependent regulationonly has to compensate for fluctuations in rate. In this way it isreliably ensured that the evaporation rate is also largely maintainedconstant, even though the regulation system, in contrast to the knownrate regulation systems, would immediately respond to any change in thevacuum or oxygen partial pressure.

The so-called getter effect is largely compensated for by the featureaccording to the invention, which arises on account of the fact thatsome of the oxygen molecules are removed by a pure getter process fromthe vacuum deposition process by the precipitation of oxides.

The method according to the invention can also be used in the case ofsubstances that additionally liberate oxygen in a reactive atmosphere,such as e.g. quartz. In this case the sign of the regulating signal issimply reversed, i.e. an increase in the pressure or partial pressureleads in this case to a reduction in the evaporation rate.

BRIEF DESCRIPTION OF THE DRAWING

One embodiment of an apparatus for carrying out the method according tothe invention will be described in more detail hereinafter with respectto the accompanying FIGURE which shows a perspective view of the vacuumchamber and a schematic representation of the control system therefor.

DETAILED DESCRIPTION OF THE INVENTION

A vacuum deposition chamber 1 is provided on its front side with asealing flange 2 and sealing strips 3, an appropriate counter flange ofa door (not shown) being in contact therewith during operation. Apipeline 6 passes via a duct 5 through a cylindrical chamber wall 4, thepipeline being connected via a gasmeter 7 to an oxygen source 8. Anautomatically regulated metering valve 9 whose throughput can beadjusted by means of an adjustment knob 10 is located in the pipeline 6.

A suction pipe 12 of a vacuum pump unit 13 passes through a base plate11. A spherical substrate holder 15 whose rotation shaft is fastened toa flange plate 16, is rotatably secured to an upper cover plate 14. Thesubstrate holder 15 is rotated by means of a drive motor 17.

A resistance-heated evaporator 18 together with an evaporation vessel 19is arranged beneath the substrate holder 15 on the base plate 11, theresistance of the vessel forming the so-called evaporator resistance.The evaporator 18 is connected via lines 20 to a regulable power supplyunit 21. The current flow in the evaporation vessel 19 and thus theevaporation rate can be influenced by regulating the power supply. Anelectron beam evaporator 22 with an evaporation crucible 23 and anelectron beam source 24, which latter is connected via lines 25 to alikewise regulable power supply unit 26, are also arranged on the baseplate 11 beneath the substrate holder 15. The required high voltage andalso the heating voltage for the cathode (not shown in more detail) ofthe electron beam source 24, are applied through the connecting lines25. By altering the high voltage (accelerating voltage) and/or theheating voltage, it is possible to influence the output of the electronbeam source and thus the evaporation rate of the evaporant located inthe evaporation crucible 23.

The base plate 11 also accommodates a pressure measurement sensor 27which corresponds to the pressure range and indication accuracy, andsupplies an electrical measurement signal corresponding to the oxygenpartial pressure via an electrical lead 28 to a pressure measuringdevice 29 equipped with a pressure indicator 30. The pressure measuringdevice 29 has an output 31 which is connected via an electrical lead 32to a regulator 33. The regulator 33 has an output 34 which can beconnected, preferably switched over, to a final control element orregulator 35 for the power supply unit 21, and to a final controlelement or regulator 36 for the power supply unit 26. The regulator 36suitably influences the emission current of the cathode of the electronbeam source 24.

The described apparatus operates in the following manner: by means ofthe vacuum pump unit 13, a pressure lower than 5×10⁻⁶ mbar is initiallyproduced in the vacuum deposition chamber 1. Such an amount of oxygen isthen introduced per unit time into the container by means of themetering valve 9 that a pressure of 7×10⁻⁴ mbar is established as anequilibrium state. As soon as the evaporation of oxidisable substancesbegins in the resistance-heated evaporator 18 or in the electron beamevaporator 22, the oxygen partial pressure in the vacuum depositionchamber 1 falls, and in fact to a pressure that corresponds to thedesired deposition conditions. If now this pressure rises on account ofsome process or other, for example because too much oxygen flows throughthe metering valve 9, or because the evaporation rate decreases, thenthe pressure measurement sensor 27 in conjunction with the pressuremeasuring device 29 records this pressure rise and produces via theregulator 33 and the final control element 35 or 36 a correction whichcan be applied via the electrical leads 37 or 38 to the power supplyunit 21 or 26. This now regulates the relevant evaporation process inthe sense that the evaporation rate is increased, whereupon an increasedoxygen requirement is established, which corresponds to the amount ofoxygen added per unit time. On the other hand, a drop in pressure in thevacuum deposition chamber 1 results in a decrease in the depositionrate, with the result that this again corresponds to the reduced oxygensupply.

EXAMPLE

In an apparatus according to the FIGURE interference layer systems wereproduced in a reactive atmosphere on glass substrates by alternatedeposition of SiO₂ and TiO₂. In this case a double-cup crucible in whichthe two deposition materials were accommodated separately was held inthe electron beam evaporator 22. By rotating the evaporation crucible 23one of the evaporation substances could be brought into the path of theelectron beam emitted from the electron beam source 24. An oxygenpressure of 7×10⁻⁴ mbar was established by means of the metering valve9, after the vacuum deposition chamber 1 had firstly been evacuated to apressure of 3×10⁻⁶ mbar. The deposition materials were first of allmelted under a diaphragm or aperture, not shown. The emission current ofthe electron beam source 24 was thereby initially limited by a presetboundary value. After the melting process, a predetermined oxygenpartial pressure was established by means of the final control element36 via the emission current. The regulator 33 now held the ratio ofoxygen partial pressure to the evaporation rate constant in theafore-mentioned regulation sense. As soon as a layer of one of the layermaterials had been deposited with a layer thickness of a quarterwavelength, the next following layer was produced in a similar mannerwith the second deposition material. Since the desired value of thepressure differed with respect to the different materials, this fact wastaken into account by means of a different predetermined desiredpressure value. The individual steps of the deposition process wereterminated by turning the diaphragm, not shown. The diaphragm itself wascontrolled by a photometer, likewise not illustrated, as a function ofthe layer thickness. Layer thickness measurement techniques anddiaphragm control are however part of the state of the art, andaccordingly do not need to be discussed in more detail here.

The layers produced are characterized both individually and in theirtotality as layer systems by a high degree of homogeneity and a verygood reproducibility with different batches.

We claim:
 1. In a process for regulating the evaporation rate ofoxidisable substances in reactive vacuum deposition by the meteredaddition of oxygen to a vacuum deposition chamber during deposition, theimprovement comprising: adjusting the addition of oxygen to apredetermined value, measuring the pressure in the vacuum chamber, andregulating the evaporation rate for any pressure changes such that theratio of pressure to evaporation rate is maintained substantiallyconstant.
 2. The method according to claim 1, wherein the adjusting ofthe addition of oxygen comprises regulating the amount of oxygen to aconstant value per unit time.
 3. The method according to claim 1, in aresistance-heated evaporator, further comprising regulating the currentflowing through the evaporator resistance in a pressure-dependentmanner.
 4. The method according to claim 1, in an electron beamevaporator, further comprising regulating the emission current of thecathode in a pressure-dependent manner.
 5. The method according to claim1, further comprising measuring the oxygen partial pressure.